It is known to automatically control windshield wipers on a vehicle by using a precipitation sensor coupled to a wiper system to actuate the system when rain or other water is detected. In some cases the sensor is on the windshield surface and wiped during wiper operation. Usually such sensors are resistive or capacitive. The effect of the water drops on the resistance or capacitance is a function of the purity of the water. For example, if a pair of interdigitated electrodes are bridged by a few drops of water the resistance across the electrodes is a linear function of the number of drops but also depends on the resistivity of the water. The resistivity may be quite high, in the case of relatively pure rain, or quite low in the case of rain or road spray containing salts so that the sensitivity may depend more on the resistivity than on the number of drops or amount of surface coverage. Thus a circuit sensitive to a given resistance across the electrodes may be actuated by much or little water depending on the water's characteristics, and thus cannot be calibrated for a given amount of water for consistent or repeatable operation. In addition, such sensors are insensitive to small amounts of water while proper wiper operation depends on detecting a very small percentage of water coverage on the windshield.
It is desired, however, to detect a given number of water drops in a sensor area or a given percentage of area coverage independently of water resistivity. Preferably the sensor is integrated into the windshield wiper system in a manner to respond to the conditions prevalent across the windshield.